Sheath-core conjugate fibers produced by high-speed melt spinning are known. For example, JP-B-54-38214 discloses a process of producing a conjugate fiber, in which a fiber-forming crystalline polymer as a core component and a polymer having a softening point lower than the softening point of the crystalline polymer by at least 40° C. as a sheath component are spun in a sheath-core configuration at a sheath component weight ratio of 5 to 75% and taken up at a speed of 3200 to 9800 m/min.
The publication alleges that the conjugate fiber obtained by the process has reduced heat shrinkage. However, the actual heat shrinkage (boiling water shrinkage) percentages are in a range of from 12.7% to 37.2%, which cannot be seen as sufficiently small to manufacture nonwoven fabric by thermally bonding the fibers at the intersections. The publication mentions that the conjugate fiber mass can be opened using air to form a web and that the conjugate fiber can be converted into staple as a material of staple nonwoven fabric but gives no considerations to web formation using a card.
Various proposals have been made to manufacture nonwoven fabric with improvements in bulk, strength, and feel by using conjugate fibers. For instance, JP-A-8-60441 proposes using a three-dimensionally crimped heat fusible conjugate fiber composed of crystalline polypropylene as a first component and polyethylene as a second component to provide a nonwoven fabric with improved strength and recovery of bulk. JP-A-11-323663 proposes using a heat fusible conjugate fiber the cross-section of which is modified to have branches extending in strands to provide nonwoven fabric with a pleasant feel. JP-A-2001-3253 proposes a bulky nonwoven fabric having (1) thermobonded parts in which heat fusible conjugate fibers are thermally bonded but are not compressed nor flattened and (2) non-thermobonded parts. However, since bulkiness or feel and strength of nonwoven fabric conflict with each other, there has been no nonwoven fabric satisfying all these requirements.